A polymeric porous film or sheet having a large number of micro-pores is utilized in various fields as separation films for use in the production of ultra-pure water, the formation of chemicals, water treatment; waterproof moisture-permeable films for use in cloths, sanitary materials, and the like; and a separator for a battery.
As this kind of techniques for forming a large number of interconnected micro-pores in high polymers, various techniques described below are proposed.
For example, proposed in Japanese Patent Application Laid-Open No. 5-25305 (patent document 1) is the method of obtaining a porous film by kneading ultra-high-molecular-weight polyethylene and a solvent to form the mixture into a sheet, stretching the sheet, and extracting the solvent.
In the above-described method, as described at the paragraph number [0045] of the specification, because the solvent is extracted by being cleaned with the organic solvent for cleaning use, a large amount of the organic solvent is necessary, which is unpreferable for environment.
Proposed in Japanese patent No. 3166279 (patent document 2) is the method for obtaining an interconnectable porous film or sheet by inflation-molding the resin composition containing the polyolefin resin and the filler and mono-axially stretching the obtained film or sheet in the take-off direction.
Also in Japanese Patent Application Laid-Open No. 2004-95550 (patent document 3), there is disclosed the porous film which is used as a separator for a lithium secondary battery. A sheet formed by molding the resin composition containing the thermoplastic resin and the filler is stretched at least mono-axially to obtain the porous film.
Because the filler is present in the surface of the porous films or sheets obtained in the above-described methods, irregularities are formed to a proper degree. Thus the film has a high sliding performance. But because the filler is present in all the layers, the mass thereof per unit area (basis weight) is large. Therefore there is room for improvement.
To keep the surface roughness of a porous film or sheet to some extent and make the basis weight small, in the porous film made of polyethylene resin disclosed in Japanese Patent Application Laid-Open No. 11-060792 (patent document 4), a surface roughening agent consisting of finely divided particles such as the filler is contained in only the surface thereof (claims 11, 12 and paragraph 0018).
But in the production of the porous film, the film is made porous by removing the plasticizer (claims 10 to 12). Similarly to the invention described in the patent document 1, a large amount of an organic solvent is necessary to remove the plasticizer. Therefore there is room for improvement to reduce a load to be applied to environment.
Proposed in Japanese Patent Application Laid-Open No. 10-50286 (patent document 5) is the porous film which is produced by heat-treating the film made of polyolefin, having a high melting point and the film made of the polyolefin having a low melting point to adjust birefringence and elastic recovery thereof, obtaining a laminate film having not less than three layers integrated with each other by thermal compression bonding, stretching the laminate film at two steps to make the laminate film porous, and performing thermal fixing so that the obtained porous film is used as a separator for a battery.
In a method called an open pore stretching method of forming pores through a single polymer, it is necessary to produce a preferable porous in a very narrow structure stretching condition (paragraphs [0025] through [0028]) including the stretching temperature, the ratio of a stretched dimension to an original dimension, the multi-stage stretching, and the like. Therefore it is unpreferable to produce the porous film by using the above-described method when considering a process management for producing it in the industrial scale.
A foaming technique of using a subcritical fluid or a supercritical fluid is known. More specifically, a polymer is impregnated with the subcritical fluid or the supercritical fluid to obtain a saturated state. Thereafter a super-saturated state is generated by rapidly reducing a pressure or the like to utilize foaming of a super-saturated gas.
The above-described method has advantages of providing fine and homogeneous foaming and applying little load to environment when an inert gas such as carbon dioxide or nitrogen is used as the subcritical fluid or the supercritical fluid.
But in the neighborhood of the surface of the polymer, when the pressure decreases suddenly, the super-saturated state is not generated but the gas is immediately discharged from the surface of the polymer owing to diffusion and vaporization thereof. Thus a region in which foaming is not generated, namely, a so-called skin layer is invariably present. Therefore it is impossible to form a porous laminate having micro-pores interconnected with each other in the thickness direction thereof.
Patent document 1: Japanese Patent Application Laid-Open No. 5-25305
Patent document 2: Japanese Patent No. 3166279
Patent document 3: Japanese Patent Application Laid-Open No. 2004-95550
Patent document 4: Japanese Patent Application Laid-Open No. 11-060792
Patent document 5: Japanese Patent Application Laid-Open No. 10-50286